Data validation rule is not working with query(importrange in Google Sheets

I'm importing data from one spreadsheet to another one: my formula =QUERY(IMPORTRANGE is in C9. Everything is working except my script regarding a data validation rule. I want a checkbox to appear in column B any time there is a data in column C.

function onEdit(event) {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var s = event.source.getActiveSheet();
  var r = event.source.getActiveRange();

 // When a client is mentioned in Col3, generate automatically the data validation for the row   
  var lastRow = s.getLastRow();
    if (r.getRow() == lastRow && r.getColumn() == 3){
    var rule = SpreadsheetApp.newDataValidation().requireCheckbox().build()
    s.getRange(lastRow,2).setDataValidation(rule); // rule to create the check box data validation
  }
 }

If I add manually content in column C, the script works. But it doesn't work for the imported data. Does anyone have an idea?

Thank you for your help!